1. Field of the Invention
The present invention relates to a display device and more particularly, a plasma display device and a driving method thereof.
2. Background of the Related Art
In general, a plasma display panel is formed of unit cells, and each unit cell includes a front substrate, a rear substrate and a barrier rib or a partition formed between the substrates. Each cell is filled with an inert gas mixture containing neon (Ne), helium (He) or major discharge gases such as a mixed gas of Ne+He, and a small amount of xenon. When discharge occurs by a radio frequency voltage, the inert gas generates vacuum ultraviolet rays and irradiates fluorescent substances formed between barrier ribs to display an image. The plasma display panel is thin and light.
FIG. 1 illustrates the image gradation processing method used in a plasma display panel. According to the gray level of an image, a frame is divided into a plurality of subfields of different number of luminescence. Each subfield is composed of a reset period (RPD) for initializing (or resetting) all cells, an address period (APD) for selecting a cell to be discharged, and a sustain period (SPD) for implementing gray level by a number of discharges. For instance, if an image is displayed in 256 gray levels, a frame period (16.67 ms) corresponding to 1/60 sec is divided into 8 subfields SF1-SF8, and each of the subfields SF1-SF8 is subdivided into a reset period, an address period, and a sustain period.
The reset period and the address period are uniformly set for every subfield. The address discharge for selecting a cell to be discharged arises by potential difference between the address electrode and the scan electrode. The sustain period in each subfield increases at the rate of 2n (n=0, 1, 2, 3, 4, 5, 6, 7). Since the sustain period changes in each subfield, the sustain period of each subfield, that is, the number of sustain discharges, can be adjusted to express an image in gray level.
FIG. 2 is an illustration of driving waveforms for a plasma display panel. The operation of the plasma display panel is performed using four periods in each subfield as follows: a reset period for initializing all the cells, an address period for selecting a cell to be discharged, a sustain period for sustaining discharge of the selected cell, and an erase period for erasing wall charged formed in the discharged cell.
A rising ramp waveform (Ramp-up) is simultaneously applied to all the scan electrodes in the set-up interval of the reset period. The rising ramp waveform (Ramp-up) causes a weak dark discharge within discharge cells. By the set-up discharge, wall charges with straight polarity (e.g., positive voltage) are accumulated on the address electrode and the sustain electrode, and wall charges with reverse polarity (e.g., negative voltage) are accumulated on the scan electrode.
In the set-down interval of the reset period, a falling ramp waveform (Ramp-down) falling from a positive voltage lower than a peak voltage of the rising ramp waveform (Ramp-up) to a specific voltage level, preferably lower than a ground (GND) voltage level, causes a weak erasure discharge within the cells, to thereby erase excessively formed wall charges on the scan electrode. The set-down discharge uniformly leaves wall charges required for the stable address discharge within the cells.
In the address period, a negative scan signal is sequentially applied to the scan electrodes, and a positive data signal is applied to the address electrode synchronously with the scan signal. A potential difference between the scan signal and the data signal adds to a wall voltage generated during the reset period, to generate an address discharge within the discharge cells to which the data signal is applied. The wall charges are formed within the cells selected by the address discharge, in order to cause discharge when a sustain voltage Vs is provided during the sustain period. In the meantime, a positive voltage Vz is provided to the sustain electrode (Z) during the set-down interval and the address period, in order to reduce the potential difference with the scan electrode, thereby preventing erroneous discharge with the scan electrode.
In the sustain period, a sustain signal Sus is alternately applied to the scan electrodes and the sustain electrodes. The wall voltage within the cell selected by the address discharge is added to the sustain signal, and hence, a sustain discharge, i.e., display discharge, is generated between the scan electrode and the sustain electrode every time a sustain signal is applied to either the scan electrode Y or the sustain electrode Z. After the sustain discharge, a voltage of an erasing ramp waveform (Ramp-ers) having a small signal width and a low voltage level is provided to the sustain electrode to thereby erase remaining wall charges within the cells.
In case of a plasma display panel driven by the above-described driving waveform, in the address period, the scan signal and the data signal are concurrently applied to the corresponding scan electrodes and the address electrodes X1-Xn. FIG. 3 is an illustration of a timing chart of signals applied to corresponding selected scan electrode Ym and address electrodes X1-Xn in the address period.
As shown in FIG. 3, in the address period, the corresponding data signals are applied to the address electrodes X1-Xn concurrently (i.e., at ts) with the scan signal provided to a selected scan electrode for selecting the corresponding cells in a row of the plasma display device. When the corresponding data signals and the scan signal are applied simultaneously to the address electrodes X1-Xn and the scan electrode, respectively, noises are generated in a waveform applied to the scan electrode and a waveform applied to the sustain electrode. FIG. 4 is an explanatory diagram of the problems caused by signals provided to the address electrode and the scan electrode during the address period.
If data signals and a scan signal are applied to the corresponding address electrodes X1-Xn and the scan electrode, respectively, noises are generated in the waveforms. In general, these noises are generated because of the coupling of panels through capacitance. When a data signal rises rapidly, noises rise in the waveforms being applied to the scan electrode and the sustain electrode. Similarly, when a data signal falls rapidly, noises also fall in the waveforms being applied to the scan electrode and the sustain electrode. These noises make the address discharge occurred in the address period unstable, and reduces the driving efficiency of the plasma display panel.
In general, the above driving waveform often generates erroneous discharge when the temperature of the panel is high or low. Erroneous discharge caused by a high ambient temperature of the panel is called a high-temperature erroneous discharge, and erroneous discharge caused by a low ambient temperature of the panel is called a low-temperature erroneous discharge.
FIG. 5 is an explanatory diagram of the high-temperature erroneous discharge in a plasma display panel driven caused by the driving waveform. If the temperature around the panel is relatively high, the recoupling rate or recombination rate between space charges 701 and wall charges 700 within a discharge cell increases. The space charges 701 are charges existing in the space within the discharge cell, and unlike the wall charges 700, space charges 701 do not participate in the discharge. In result, the absolute amount of wall charges participating in a discharge is reduced, and erroneous discharge occurs.
For example, if the recoupling rate between the space charges 701 and the wall charges 700 is increased in the address period, the amount of wall charges 700 participating in the address discharge is reduced, resulting in an unstable address discharge. In this case, the address discharge becomes even more unstable because there is enough time for recoupling between the space charges 701 and the wall charges 700 in the latter half of addressing. Therefore, a discharge cell that was turned on in the address period may be turned off in the sustain period (i.e., the high-temperature erroneous discharge).
Moreover, if the temperature around the panel is relatively high and a sustain discharge occurs in the sustain period, the space charges 701 move faster during the discharge, so more space charges 701 are recoupled with the wall charges 700. Thus, after any sustain discharge, the amount of wall charges 700 participating in the sustain discharge is reduced due to the recoupling or recombination between the space charges 701 and the wall charges 700. In consequence, a next sustain discharge may not be generated at all (i.e., the high-temperature erroneous discharge).
FIG. 6 is an explanatory diagram of the low-temperature erroneous discharge caused by the driving waveform. If the temperature around the panel is relatively low, heat energy supplied into a discharge cell is reduced. Thus, the absolute amount of seed electrons that collide with neutrons for producing other electrons is decreased, resulting in erroneous discharge. According to the plasma discharge mechanism, a predetermined energy, e.g., heat energy, inside a discharge cell is applied to a certain seed electron. Then, the seed electron is accelerated by the energy, and collides with a neutron. The same neutron emits an electron as a result of the collision, and the emitted electron collides with another neutron for emitting still another electron. In this manner, plasma discharge is generated.
However, if the temperature around the plasma display panel generating the plasma discharge becomes relatively low, the amount of heat energy to be applied to a seed electron is reduced. Accordingly, the plasma discharge mechanism cannot be operated smoothly. That is, the plasma discharge mechanism slows down and the erroneous discharge occurs. For instance, the address discharge does not occur in the address period due to the reduction of heat energy. Hence, a discharge cell that needs to be turned on in the sustain period is often turned off (i.e., the low-temperature erroneous discharge).
The above descriptions are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.